Measurement of bainite packet size and its influence on cleavage fracture in a medium carbon bainitic steel

For a bainitic steel of C = 0.38%, the bainitic packet size measured by OM is about 3.5 times larger than that measured by EBSD with a 15° grain boundary misorientation. This ratio is used to calculate the effective surface energy given by Griffithʹs model for cleavage fracture. Average ferrite lath width and cementite carbide width values are also measured. Following the general form of Griffithʹs equation, the effective surface area of cleavage fracture γp is determined for each microstructural unit. It is concluded that the bainite packet size determined by EBSD with a 15° misorientation angle criterion is the microstructural parameter that controls cleavage crack propagation. Given the relationship between the average unit crack path (UCP) and the bainitic packet size, it is concluded that the effective surface energy of cleavage fracture (γp) is between 71.6 and 82.6 J m−2.